An electronic stethoscope or contact microphone operates as a vibration sensing element contacted to a body part such as a person's skull or attached to one's neck or throat, for example, so as to detect sound propagating through tissue, muscle, tendon, ligament, and bone. In this way, an acoustic signal (e.g. a voice signal) is detected by the sensor. In conventional microphone systems, noise is frequently a problem. It is particularly troublesome in high noise environments, for example, in factories, construction sites, engine rooms, and the like.
In medical diagnostic systems employing acoustic sensors, a typical acoustic device is a stethoscope which operates to detect sound generated in internal organs. Its ability to direct sound transmission to a sensing element without interference from external noise is very important. Other medical applications include continuous monitoring of a patient's acoustic signals, for example, in order to quickly detect and alert medical personnel to any abnormal signals indicative of a patient's health.
Conventional contact microphones utilize a structure comprising a rubber cylinder of diameter D, having a flat, top end surface and a bottom end surface opposite one another. A layer of polyvinylidene fluoride (PVDF) film is wrapped around the curved side of the cylinder. The first end surface of the rubber is contacted to the skin while the other is connected to a holder. When vibrations are transmitted to the contacted end, the diameter of the rubber cylinder expands or shrinks depending on the vibration. The PVDF material surrounding the cylinder detects the diameter variation. However, the process of wrapping PVDF onto a cylinder and bonding to it to form such an acoustic device is a difficult task in terms of mass producing these devices within predetermined working requirements and tolerances. Consequently, the cost of producing such devices becomes expensive. A need exists to obtain a low cost and efficient method of forming a device having a sensitivity similar to that of conventional contact microphones formed of cylindrical rubber surrounded or wrapped by a layer of PVDF.
Conventional contact microphones are also formed having a thick layer of PVDF-TrFE copolymer used as the piezoelectric element. A backside of the layer is bonded to a plate which acts as a mass. The front side of the copolymer layer is directly contacted to the skin of a person. Vibrations transferred through the skin induce pressure variations to the sensor such that the thickness of the copolymer layer is expanded or contracted, thereby generating a voltage signal. In order to obtain the necessary sensitivity for medical applications, the thickness of the copolymer layer has be on the order of a few millimeters. A significant drawback associated with this type of device is that the cost to produce the thick copolymer layer is much higher than that of PVDF film.
An apparatus having a sensitivity which is similar to or greater than that of the thick copolymer layer and having a production cost substantially lower than conventional devices is highly desired. It is also desired to obtain an apparatus that is less susceptible to acoustic interference than conventional contact microphones.